1. Field of the Invention
The present invention relates to an anode that contains an anode active material containing silicon (Si) as an element and a secondary battery including the anode.
2. Description of the Related Art
In recent years, many portable electronic devices such as a combination camera (video tape recorder), a digital still camera, a mobile phone, a personal digital assistance, and a notebook personal computer have been introduced, and down sizing and weight saving thereof have been made. Accordingly, development of a light-weight secondary battery capable of providing a high energy density as a power source for such electronic devices has been promoted. Specially, a lithium ion secondary battery in which a carbon material is used for the anode, a complex material composed of lithium (Li) and a transition metal is used for the cathode, and ester carbonate is used for an electrolytic solution provides a higher energy density compared to existing lead batteries and nickel cadmium batteries, and thus the lithium ion secondary battery is in practical use widely.
Further, in recent years, as performance of portable electronic devices has been improved, further improvement of the capacity has been demanded. It has been considered that as an anode active material, tin, silicon or the like is used instead of the carbon materials as described in, for example, U.S. Pat. No. 4,950,566. The theoretical capacity of tin is 994 mAh/g, and the theoretical capacity of silicon is 4199 mAh/g, which are significantly larger than the theoretical capacity of graphite 372 mAh/g, and therefore capacity improvement is able to be expected therewith.
However, a tin alloy or a silicon alloy inserting lithium has a high activity. Therefore, there have been disadvantages that an electrolytic solution is easily decomposed, and lithium is easily inactivated. Therefore, when charge and discharge are repeated, charge and discharge efficiency is lowered, and sufficient cycle characteristics may not be obtained.
Therefore, it has been considered that an inert layer is formed on the surface of the anode active material. For example, it has been proposed to form a silicon oxide coat on the surface of the anode active material as described in Japanese Unexamined Patent Application Publication Nos. 2004-171874 and 2004-319469.
Further, the anode active material containing tin, silicon or the like expands and shrinks more largely than an anode active material composed of a carbon material such as graphite due to repetition of charge and discharge. Therefore, collapse of the anode active material itself, separation of the anode active material from the anode current collector and the like may result in deterioration of the cycle characteristics.
To address the foregoing disadvantages, a technique in which the cycle characteristics are improved by adopting an electrode using, as an active material, an amorphous material containing at least one impurity selected from the group consisting of carbon, oxygen, nitrogen, argon, and fluorine together with silicon have been proposed as described in, for example, Japanese Unexamined Patent Application Publication No. 2005-235397. As a technique similar thereto, Japanese Unexamined Patent Application Publication No. 2007-184252 discloses an active material having a composition expressed by general formula SiCxOy (x: 0.05 to 0.90, y: 0 to 0.9).